A person that is closer to you and that knows you better than your normal friends.
A person who will always be there for you. You can literally count on anytime of day. For example, you can randomly call them at 2 a.m. and they'll pick up and talk to you all night if you need to be cheered up.
A person who knows just what to say to make you laugh or smile on the days when you don't want to.
A person who knows you the way they know the back of their hand.
A person you can talk to about everything and anything, from the most random funny topics to the most serious topics. You can be totally honest with this person.
A person you don't have to act fake around because they accept you for who you are, no matter how lame you are.
guy: You're my besterestest friend!
girl: Aww, you're my besterestest friend too!